German Offenlegungsschrift 2,329,300 relates to heterocyclic polyisocyanates obtained by reacting diisocyanates with hydrocyanic acid. In addition to hydrocyanic acid, compounds which eliminate hydrocyanic acid, such as, for example, the addition products of hydrocyanic acid with aldehydes or ketones (cyanhydrins), are also recommended as starting materials. The structure of the polyisocyanates obtained by the process according to DT-OS 2,329,300 is independent of whether hydrocyanic acid or the above-mentioned hydrocyanic acid derivatives are used as starting material (Example 10 of DT-OS 2,329,300). This discovery can be attributed to the fact that the authors of DT-OS 2,329,300 used reaction conditions under which the hydrocyanic acid adducts with aldehydes or ketones decomposed into their constituents, hydrocyanic acid and aldehyde or ketone, before reaction with the diisocyanate.
It has now surprisingly been found that new isocyanates having advantageous properties by comparison with the isocyanates of the above-mentioned prior art can be obtained by carrying out the reaction between diisocyanate and cyanhydrins in a first reaction stage under such mild conditions that the cyanhydrin is not decomposed into hydrocyanic acid and carbonyl compound, but instead a simple adduct of the cyanhydrin with the diisocyanate is initially formed. The action of heat on this intermediate product in the presence of excess quantities of starting diisocyanate results in the formation of new heterocyclic isocyanates corresponding to general formula (I) below (n=O, Y=--O--). These new isocyanates are distinguished from the isocyanates according to DT-OS 2,329,300 obtained from the corresponding starting materials in particular by their much lower viscosity and by the better lacquer properties of the polyurethane lacquers produced from them.
According to the invention it has also been found that isocyanates which are largely similar in structure and properties, and which, in particular, have valuable lacquer properties, are formed from organic diisocyanates by a similar reaction with .alpha.-aminonitriles, .beta.-hydroxy or .beta.-aminonitriles.